1. Field of the Invention
This invention relates to a process for producing a polyester film having a thin cured coating of polyurethane, and to the use of the polyester film for the production of a printed film by applying an ultraviolet-curable ink. More specifically, it relates to a process for producing a polyester film which has solvent resistance, antiblocking property and ease of adhesion, particularly excellent adhesion to a radical-polymerized resin and is useful as a substrate for film products, particularly a floppy disc, a magnetic card, a membrane circuit, an adhesive tape, a label, a dry film resist and a masking film, and to the use of the polyester film.
2. Discussion of the Prior Art
It is known to form films by melt-extrusion of thermoplastic polyesters such as polyethylene terephthalate or a copolymer thereof, polyethylene naphthalate or a copolymer thereof, or a blend of such a polymer or copolymer with a minor proportion of another resin. Biaxially stretched and heat-set polyester films finally obtained are known to have better thermal resistance, gas-barrier property, electrical properties and chemical resistance than films of other resins. Since, however, the surface of such polyester films is highly crystallized and oriented, it has high cohesiveness and therefore poor adhesion to paints, adhesives, inks, etc.
Physical treatments (such as corona discharge treatment, ultraviolet light treatment, plasma treatment), electron beam treatment, flame treatment, or chemical treatments (treatments with chemicals such as alkalies, aqueous amine solutions, trichloroacetic acid, and phenols) are known as a method of improving the adhesion. However, these methods are disadvantageous in practical application because they may cause degradation of the adhesion with time or the working environment may be polluted by the volatilization of the chemicals.
Another known means is to form a primer layer on the film surface by coating an easily adhesive coating agent in an ordinary polyester film-producing step, or in another step.
Japanese Laid-Open Patent Publication No. 6767/1982 (U.S. Pat. No. 4,364,885 and British Pat. No. 2,079,182) discloses an easily adhesive polyester film comprising a polyester film and coated on its surface, a composition comprising a montan wax salt and an aqueous dispersion of polyurethane.
Japanese Laid-Open Patent Publication No. 90022/1982 discloses a method which comprises coating the surface of a plastic film with a composition composed of a polyol compound having an ethylenically unsaturated bond and a polyisocyanate compound in an NCO/OH mole ratio of at least 0.8 and curing the composition to form a layer of polyurethane resin having an ethylenically unsaturated bond. This patent document describes diols having a relatively low molecular weight obtained by reacting a low-molecular-weight diepoxide with 2 equivalents of acrylic or methacrylic acid and hydroxylterminated polybutadiene compounds as the polyol compound having ethylenically unsaturated bonds. By this method, the diol and the polyisocyanate are slowly reacted on the film surface, as can be seen from the statement that the above-mentioned curing reaction is carried out at 10 to 60.degree. C. for 2 to 3 days.
Japanese Laid-Open Patent Publication No. 78761/1983 discloses an easily adhesive polyester film obtained by coating at least one surface of a polyester film with a water-soluble or water-dispersible polyester resin having a sulfonic acid group and a water-soluble or water-dispersible polyurethane resin having an anionic group. This patent document is quite silent on a polyurethane resin having an ethylenically unsaturated bond.
Japanese Laid-Open Patent Publication No. 145232/1986 discloses a method of producing an easily adhesive polyester film which comprises coating at least one surface of a polyester film with an aqueous solution of a hydrophilic self-crosslinkable polyurethane composition having blocked isocyanate groups in the molecule, and then heating the coating to cure the composition. This patent document only describes a method of imparting hydrophilicity by blocking the isocyanate groups at the chain ends of the polymer with a bisulfite and sodium sulfite. It does not disclose polyurethane containing hydrophilic groups pendant to the polymer chain. When heated for curing of polyurethane, the isocyanate groups blocked with the bisulfite or sodium sulfite liberate the bisulfite or sodium sulfite. These compounds have the defect of corroding a metal surface and rendering the coated film surface non-uniform
Japanese Laid-Open Patent Publication No. 59635/1987 discloses a method of producing a polyester film for use in magnetic cards, which comprises coating a polyurethane resin having an anionic group in an aqueous medium on at least one surface of a polyester film containing a white pigment, and stretching the coated film. This patent document is quite silent on a polyurethane resin containing a pendant anionic hydrophilic group in the polymer chain and having a carbon-carbon double bond. Japanese Laid-Open Patent Publication No.
59636/1987 discloses a method of producing a polyester film for wiring base boards which is substantially the same as the method of the above-cited Japanese Laid-Open Patent Publication No. 59635/1987. This patent document does not describe a polyurethane resin having a pendant anionic hydrophilic group in the polymer chain and a carbon-carbon double bond.
Japanese Laid-Open Patent Publication No. 97890/1987 discloses a label prepared by forming a printed layer of an air-curable ink, an ultraviolet-curable ink or an electron beam-curable ink on the polyurethane-coated surface of the polyester film produced by the method described in Japanese Laid-Open Patent Publication Nos. 59635/1987 or 59636/1987.
Japanese Laid-Open Patent Publication No. 173253/1987 discloses a method of producing a polyester film for wiring boards, which comprises coating a coating composition comprising a polyurethane having an anionic group, a crosslinking agent and fine particles on a polyester film and then stretching the coated film. This patent document neither discloses polyurethane having a pendant anionic hydrophilic group in the polymer chain and a carbon-carbon double bond.
Japanese Laid-Open Patent Publication No. 263237/1987 discloses a method of producing a surface-cured polyester film, which comprises forming an actinic ray-curable acrylic resin layer on the polyurethane-coated surface of the polyester film produced by the method of Japanese Laid-Open Patent Publication Nos. 59635/1987 or 59636/987.